This invention pertains to the art of microwave electrical heating and, more particularly, to an apparatus and a method for using microwave energy to heat frozen goods packaged in a metallic container.
Frozen foods, particularly frozen prepared convenience foods, form a significant portion of the groceries bought by many people. In the past and presently, many of these frozen foods have been marketed in aluminum containers or other microwave opaque structures. These aluminum containers or trays provide excellent shipping containers, freezing containers and cooking containers for use in conventional ovens. These and other desirable features of aluminum trays, including economy, have made them the overwhelming favorite among food processors as containers for processed foods sold to consumers.
The reconstitution of frozen prepared foods requires raising the temperature throughout the food generally to about 180.degree. F. Raising the temperature of portions of the food above 180.degree. F. wastes heat and can damage the food item by drying it out. Some over-heating of food portions is normally required in reconstituting frozen foods in order that heat will be conducted from hot outer portions to cooler inner portions. However, excessive overheating is damaging and wasteful.
Present microwave ovens normally consist of an enclosed cooking area which will contain microwave energy thereby protecting the consumer. Microwave energy is generated in the 915 megahertz or 2,450 megahertz bands by a magnetron and introduced into the microwave containing area. The microwave energy penetrates food substances and is normally absorbed within about one inch of the surface of most foods. The absorbed energy heats the food and this heat is transferred to other portions of the food by conduction to heat the entire food item.
Metallic containers, such as the metallic trays presently used with frozen prepared foods, present significant problems when used in microwave ovens. Metallic containers are opaque to microwaves. Therefore, foods contained within a metallic container can only be heated through surfaces which are not covered by the metal, normally the top only, and not through the remaining surfaces of the food item. This results in uneven heating, with some portions being dried out or overcooked and other portions remaining cold or unreconstituted.
Metallic surfaces in microwave ovens also result in "arcing" which can damage the metallic surface on which the arcing occurs and adjacent surfaces such as the food item. Arcing can also damage the microwave oven itself. Heretofore, microwave oven manufacturers have recommended that metallic substances or surfaces not be introduced into microwave ovens except in very specifically defined instances because of these and other problems.
Various structures have been proposed for heating food in a microwave oven using metallic trays as part of a container. U.S. Pat. No. 4,013,798 to Goltos describes a plastic tray containing food which is placed inside a metallic shield allowing one to heat different types of food at different rates at the same time. U.S. Pat. No. 3,731,037; No. 3,854,023; and No. 3,985,990 all to Levinson describe various complex structures including microwave absorptive material which is heated in a microwave oven. The heat generated in the microwave absorptive material is contained within the structure and used to heat the contained food. None of these structures will heat food contained in a metallic tray evenly through all surfaces.
Other methods of heating using microwave ovens are described in U.S. Pat. No. 3,753,651; No. 4,091,119; No. 4,178,494 and German Pat. No. 2,526,196. U.S. Pat. No. 3,745,290 and British Pat. No. 622,013 describe electric means other than microwaves for heating food.
The present invention contemplates a new and improved apparatus and method for heating food filled metallic trays in a microwave oven and heating the food contained in the tray from all surfaces which overcomes the above referred to problems and others and enables food processors to package frozen prepared food in a container usable in all of the major cooking techniques.